Solar parabolic troughs, which focus sunlight on tubes carrying a fluid that conveys heat for steam generation (e.g. to a steam-driven electric generator, or for industrial process heat) or to a body of material for energy storage, are a proven, reliable, and relatively low-cost technology for collecting energy. The tubes upon which the light is focused in such systems are typically termed “receiver tubes,” “receivers,” or “heat collection elements.” At a typical electric generation plant employing solar parabolic troughs, many receivers (e.g., thousands) are arrayed with reflective troughs in parallel rows to form a “field” that can collect sufficient energy for a generating system of economical size. At present, receivers represent approximately 12% of the capital cost of a concentrating solar power installation employing solar parabolic troughs.
In a typical receiver constructed according to the prior art, a central liquid-carrying tube with an outer optical absorption coating is surrounded by a vacuum held within a transparent concentric envelope. Light focused on the receiver by a mirror (also known as a “collector”) passes through the transparent concentric envelope and through the vacuum and impinges on the central liquid-carrying tube. The coating on the central tube absorbs most (preferably all, although this cannot be realized in practice) of the energy incident upon it and is thus heated. This heat is transmitted by conduction through the wall of the central tube and thence to the tube's liquid contents. The heated liquid is pumped through the receiver and, in general, through additional receivers, being thus raised to a high temperature (e.g., 400° C.) before being pumped to a boiler, or energy storage device (e.g., reservoir of hot fluid). The function of the vacuum between the inner, fluid-carrying tube and the outer, transparent envelope is to prevent loss of heat from the receiver by convection and conduction to the outer envelope and thence, by radiation and conduction, to the environment.
A number of problems in the use of standard vacuum-containing receiver tubes have been observed. These include, but are not limited to, the following: (a) The absorption coatings on the inner, fluid-carrying tube are expensive to manufacture. (b) Degradation of a receiver's vacuum entails increased thermal losses from the receiver and, if severe enough, requires replacement of the receiver. In practice, vacuum degradation causes failure of 1-5% of receiver tubes per year. (c) The tubular outer glass envelope of a conventional receiver must be thick enough to withstand the stresses imposed by containing a vacuum as well as by wind and its own weight. This strength requirement increases the cost of the envelope. (d) The absorptive coating upon the inner, fluid-containing tube of a receiver not only absorbs radiant energy but emits it, particularly in the infrared part of the spectrum. Emission losses increase with coating temperature T approximately as the fourth power of T (i.e., as T4). Energy thus emitted is for the most part lost to the environment, diminishing the receiver's efficiency. Moreover, the absorptive coating may be destroyed by sufficiently high T. Prohibitively large T4 radiation losses, coupled with high-temperature instability of the absorber coating, today prevent practical operation of solar parabolic-trough generating plants at elevated temperatures (>500° C.). Yet, for fundamental thermodynamic reasons it is more efficient to operate any thermal generating plant at higher peak T.
There is thus a need for receivers for parabolic-trough solar power that are less costly to acquire and maintain than are receivers constructed according to the prior art and that allow operation at higher temperature.